1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, and a process cartridge to be removably installed in the image forming apparatus.
2. Discussion of Related Art
An image forming apparatus such as a laser printer includes an image carrier in the form of a photoconductive drum that carries a toner image formed according to image data, and a transfer roller for transferring the toner image from the photoconductive drum to a recording medium in the form of a sheet of paper. The transfer roller is disposed adjacent to the photoconductive drum such that the outer circumferential surface of the transfer roller is held in rolling contact with the outer circumferential surface of the photoconductive drum. The toner image formed on the outer circumferential surface of the photoconductive drum is transferred to the paper sheet, by application of a bias voltage to the transfer roller, while the paper sheet is passed through a nip between the photoconductive drum and the transfer roller.
In the image forming apparatus of such a type as described above, the transfer roller is either installed in the main body of the apparatus, or built in a process cartridge that is to be removably installed in the main body. In the former case, the transfer roller is rotatably supported at the opposite end portions of its shaft by a pair of bearings fixed to the main body of the apparatus via respective springs interposed therebetween. The springs bias the respective bearings in a direction toward the photoconductive drum so that the transfer roller is forced onto the photoconductive drum. Each bearing has a groove open in the direction toward the photoconductive drum, so that the corresponding end portion of the shaft of the transfer roller is fitted into and removed from the groove. An example of the arrangement in which the transfer roller is installed in the main body is disclosed in JP-5-333721A.
In the arrangement in which the transfer roller is built in the process cartridge to be installed in the main body of the image forming apparatus, on the other hand, the transfer roller is rotatably supported at the opposite end portions of its shaft by a pair of U-shaped bearings fixed to the process cartridge. Each bearing is movable in the radial direction of the transfer roller, so that the transfer roller and the photoconductive drum are movable toward and away from each other. The main body is provided with a pusher member for abutting contact with the corresponding bearing on its side remote from the photoconductive drum, and a spring for biasing the pusher member toward the bearing, so that when the process cartridge is installed in the main body, the pusher members push the respective bearings so as to force the transfer roller into pressing contact with the photoconductive drum. When the process cartridge is removed from the main body, the pusher members are released from the bearings, to permit the transfer roller to be moved away from the photoconductive drum. An example of this arrangement is disclosed in U.S. Pat. Nos. 6,041,203A, 6,330,410B1, 6,411,789B1, 6,546,217B2, 6,690,903B2 and 2004126132A1.
When the used transfer roller installed in the main body of the apparatus is removed from the main body, for replacement with a new one, for instance, it is necessary to first remove the photoconductive drum from the main body, and then release the transfer roller shaft from the bearings, for removing the transfer roller from the main body. Thus, the removal of the transfer roller is relatively cumbersome. When the transfer roller is built in the process cartridge, the transfer roller can be relatively easily removed from the main body by removing the process cartridge from the main body. However, this arrangement requires the provision of the bearings and the pusher members for abutting contact with the bearings, unfavorably increasing the number of the required components of the image forming apparatus, and the structural complexity and difficulty of assembling and the cost of manufacture of the apparatus.